In many technical fields, a need exists for storing various liquid or gaseous media, such as compressed or liquefied gases, for extended periods of time and frequently at very high pressures. Many attempts have already been made in the past to satisfy this need by developing light weight pressurized medium containers or pressure vessels that would accommodate the pressurized medium without suffering leakage losses or structural damage.
For a variety of reasons, not the least important of which is the relatively high ratio of pressure that the vessel walls are able to withstand to the weight of a vessel of a given capacity, it has been found advantageous to give such walls a multilayer or composite structure, including an inner liner and an outer shell surrounding the liner and in intimate contact therewith. The liner is formed of a material, usually a metallic material, that is compatible with (i.e. inert with respect to) and also completely or at least highly impermeable to the medium being stored.
All-metallic pressure vessels have been disclosed, for example, in U.S. Pat. Nos. 2,127,712, 2,661,113, 3,140,006, and 4,964,524, of which all but the second one are directed to vessels of multilayer construction. In this instance, one of the purposes of the liner is to form an inert protective barrier preventing the medium from reaching through gross leakage or permeation through the liner to the outer shell and possibly damaging the shell. However, due to their considerable thickness and intimate contact or engagement with the shell, the liners of all-metallic pressure vessels generally contribute significantly to the load bearing capacity of the vessel. In classical state of the art vessel fabrication, the liner represents a significant fraction of the total weight. Experience with such and similar all-metallic pressure vessel constructions has shown, on the other hand, that they are limited in applicability because they are either too heavy (a criterion that is of paramount importance for applications where weight is at a premium, such as in outer space applications), or expensive to manufacture, or prone to failure, especially due to metallic material fatigue at weakened or stress concentration regions after having been subjected to a number of pressurization and depressurization cycles.
With the advent and development of high strength filaments such as glass, graphite, and synthetic plastic material fibers, and of materials, such as epoxy resins, capable of forming a matrix embedding such filaments and bonding them together into a composite structure, attempts have been made, some more successful than others, to use such composite materials for the outer shell of the pressure vessel. Of course, due to the high strength-to-weight ratio of such materials, the overall weight of the resulting vessel is significantly reduced relative to that of a comparable all-metallic vessel of the same capacity and pressure rating. Examples of vessels of this kind are disclosed, for example, in U.S. Pat. Nos. 2,744,043, 2,827,195, 3,943,010, 3,969,812, and 4,040,163.
For obvious reasons, a pressure vessel of any kind has to have at least one passage for establishing communication between the interior and exterior of the vessel. Inasmuch as the passage is usually to be connected to a conduit, such as a part of the piping of a spacecraft or the like, it is customary to provide the vessel with at least one stem or neck region that protrudes from the main body of the vessel and that is hollow so as to define the passage. This stem region is then equipped with means of one sort or another for attaching the conduit thereto. It will be appreciated that the stem region and the transition between the same and the main body of the vessel constitute a particularly vulnerable area of the pressure vessel, especially because the stem region introduces pertebrations in the relatively uniform global stress field and must be sufficiently thick as considered in the radial direction to permit attachment of the conduit thereto, whereas the liner need not be so dimensioned. At least partly for this reason, the aforementioned attaching means is sometimes provided on a separate fitting member that is mounted on the remainder of the pressure vessel so as to form at least a part of the stem region.
As should be apparent, it is important to assure that such a separate fitting member does not become dissociated from the remainder of the pressure vessel either under normal operating conditions that may involve a number of pressurization and depressurization cycles and/or conduit attachment and detachment operations, or even if the vessel is subjected to rough handling or abuse, such as during transport or other handling of the pressure vessel. This is not an easy task, particularly when the separate fitting member is to be used in a multilayer or composite pressure vessel construction, especially one using a relatively thin liner. To deal with this problem, it is currently customary to equip the separate fitting member with an enlarged portion or flange that is received in the interior of the main portion of the pressure vessel and has a contact surface of a configuration substantially corresponding to that of an associated surface of the main portion of the vessel, to brace itself against such associated surface and to thus maintain the fitting in place relative to the main portion. This enlarged portion is often secured to the lining at least along its periphery, such as by welding, to further improve the connection of the separate fitting to the main portion of the vessel and/or further enhance the impermeability of the interface between the fitting and the remainder of the pressure vessel.
While this solution may be acceptable or even advantageous in some applications, it has been realized that it is fraught with certain problems that make it less than a suitable candidate for more sensitive uses, such as those encountered in space travel or the like. For one, the region of the peripheral connection of the enlarged portion of the fitting to the liner in the main portion of the vessel constitutes a stress concentration area as the internal or external pressure to which the vessel is subjected and/or the temperature of the vessel wall changes, especially when the material of the fitting is different from that or those of the pressure vessel lining and/or shell, as is often the case. This is so because the enlarged portion of the fitting, on the one hand, and the corresponding region of the main portion of the pressure vessel, on the other hand, suffer different deformations due to such pressure or temperature changes. It is further compounded by the fact that the aforementioned corresponding region of the main portion of the vessel is shielded by the enlarged portion of the fitting from the pressure (and the temperature) existing in the interior of the vessel, so that it may undergo different deformation than if it were exposed to such condition(s). Either one of these factors may result in a rapid failure of the joint or seam and in attendant leakage therethrough.
This particular problem does not exist when the pressure vessel is constructed in accordance with the teachings of U.S. Pat. No. 4,905,856 wherein the liner extends all the way into the respective stem portion and the fitting is arranged around this projecting portion of the liner. In accordance with the disclosure of that patent, the fitting is held in position relative to the projecting portion of the liner, and thus to the remainder of the pressure vessel, by the very same filaments as those that form the shell of the pressure vessel, in that portions of such filaments are received in correspondingly configured outer peripheral recesses of the fitting and engage the respective projections that delimit such recesses. A perceived problem of this particular construction is that, as the filaments are stressed owing to the pressurized condition of the contents of the vessel, or as they undergo length changes during pressurization and depressurization of the vessel interior and its contents, they may and usually will undergo plastic deformation or creep thus compromising the original tautness of such filaments and, correspondingly, thus the quality of their engagement with the fitting, until the fitting is no longer firmly held in its desired position and the pressurized vessel is thereby rendered useless.